The present disclosure relates to an exhaust purifying apparatus for an internal combustion engine.
Japanese Laid-Open Patent Publication No. 2010-248925 discloses an internal combustion engine that includes a nitrogen oxide (NOx) removing catalyst, which removes nitrogen oxides in exhaust gas. In the exhaust passage of such an internal combustion engine, an adding mechanism, which adds urea water into exhaust gas, is provided. Ammonia of the urea water is adsorbed by the NOx removing catalyst. The ammonia adsorbed by the NOx removing catalyst reduces and removes NOx. The NOx removing catalyst is gradually degraded with the duration of use and fails to provide desired NOx removing efficiency after extended use.
In the exhaust purifying apparatus disclosed in the above publication, an abnormality diagnosis process of the NOx removing catalyst is performed. In the abnormality diagnosis process, the NOx removal efficiency of the NOx removing catalyst is calculated on the basis of, for example, the NOx concentration in exhaust gas that has been purified by the NOx removing catalyst. If the calculated removal efficiency is less than or equal to a predetermined value, it is determined that there is a possibility of abnormality due to degradation of the NOx removing catalyst.
Exhaust purifying apparatuses have been proposed that include an oxidation catalyst, which removes ammonia that has desorbed from the NOx removing catalyst and ammonia that has passed through the NOx removing catalyst (i.e., without being adsorbed by the NOx removing catalyst), in the exhaust passage downstream of the NOx removing catalyst. In such exhaust purifying apparatuses, if the above-described abnormality diagnosis process is performed on the basis of the NOx removal efficiency obtained from the amount of NOx contained in exhaust gas that has passed through the oxidation catalyst, the following inconvenience may occur.
That is, if unburned fuel is contained in exhaust gas flowing into the NOx removing catalyst, the unburned fuel is adsorbed by the NOx removing catalyst. Since the unburned fuel that has been adsorbed by the NOx removing catalyst is desorbed from the NOx removing catalyst in accordance with the engine operating condition, the desorbed unburned fuel flows into the oxidation catalyst with the exhaust gas. If NOx is contained in the exhaust gas containing the unburned fuel, a NOx reduction reaction occurs in the oxidation catalyst due to the unburned fuel.
Thus, even in a case in which the amount of NOx in the exhaust gas after passing through the NOx removing catalyst is not sufficiently reduced due to degradation of the NOx removing catalyst, the amount of NOx contained in the exhaust gas after passing through the oxidation catalyst is reduced when the NOx reduction reaction occurs in the oxidation catalyst due to unburned fuel as described above. As a result, the determined NOx removal efficiency is increased compared to a case in which the NOx reduction reaction did not occur. Therefore, even if an abnormality occurs due to degradation of the NOx removing catalyst, an erroneous diagnosis, in which it is determined that there is no abnormality due to degradation of the NOx removing catalyst, may possibly occur in the above-described abnormality diagnosis process.